1. Field of the Invention
The invention relates to a folding device for folding sheet goods, and more specifically to the zigzag folding of a sheet.
2. Discussion of Related Art
A folding device of this kind is known from European patent EP-B 0 156 326. In the folding device described therein, the speed of conveyance of the folding rollers during folding is always greater than the speed of conveyance of the feed means. Only when the leading edge of a sheet for folding arrives in a folding nip for the first time is the speed of conveyance of the folding rollers reduced for a short time to the speed of conveyance of the feed means. The faster-conveying folding rollers serve to tighten a sheet, which has to be folded in the bends that the sheet must traverse between the feed means and the folding nips, to ensure that the sheet is folded exactly at predetermined places.
One disadvantage of this known folding device is that slip can easily occur between superposed parts of the sheet fed through a folding nip, because the coefficient of friction between sheet surfaces is generally lower than that between a nip-forming surface and a sheet surface. One consequence of such slip is that sheet parts interconnected by a fold shift relatively to one another and, on the subsequent return, pass through the folding nip in the shifted state with the formation of an unwanted second fold in addition to a previous formed fold.